Leaving aside for the moment the whole climate change debate, there is no doubt that industry in general would benefit by reducing its consumption of energy. The chemical industry in particular has rightly put energy efficiency and conservation at the top of its agenda, with many plants paying increasing attention to ways of better managing their energy usage. Further opportunities certainly exist, as underscored by the recent launch by the U.S. Dept. of Energy (DOE) of its “Save Energy Now” initiative that targets major chemical plants, among other large industrial facilities (see p. 9).
One move that has already found support across the broad spectrum of operators, suppliers, government agencies and industry bodies is the use of adjustable speed drives (ASDs). These drives can be used to run a plant’s AC electric motors, which previously might have been operated at constant speed (see CP, June, p. 30). With the DOE estimating that some 18% of the energy consumed by the nation’s industrial motors could be saved by switching to energy efficient technologies, such as ASDs, it would seem to be an open and shut case in their favor. And, so it has proved in most applications. Occasionally, though, running an AC motor off an ASD can lead to problems that many a process engineer might not have come across before.

A tip-off
“We started to pick up on problems through a service we offer that analyzes field failures,” says Dan Snyder, director of applications engineering with SKF USA, Kulpsville, Pa., about a series of bearing noise and grease failure problems that were being reported back to the bearing manufacturer some years ago. The problems seemed directly related to some sort of electrical discharge through the bearing that, says Snyder, “manifested itself as possible arcing across the lubrication gap.”
Although the arcing tends to be isolated and localized, the effect on the bearing is “almost like a series of little lightning strikes,” he says. These “strikes” melt and retemper the internal bearing surfaces where the discharges occur, with the result that some surface material flakes away and spalls out to create noise in the bearing.

The first symptoms, however, are virtually invisible to the naked eye. The damaged surface appears dull, characterized by molten pit marks or microcraters that may be only around 5 to 8 μm in diameter, irrespective of whether they are on the inner ring, outer ring or a rolling element.

Initially, such damage — now known as the electric discharge machining (EDM) effect — was put down to the likelihood of stray currents from, for example, inadequately grounded welding work being carried out on the motor or driven device. But then more recently, says Snyder, “we started seeing the problem more and more on AC motors that had been fitted with frequency inverters for variable speed control. And the higher the frequency, the more bearing damage we were seeing.”

The typical type of damage only really shows up when characteristic “bearing fluting” becomes visible (Figure 1). This is caused by the dynamic effect of the rolling elements continually going over the microcraters and etching a rhythmic pattern into the running surfaces of the bearing’s races. Noise and vibration from the bearing increases, and eventually the deterioration will lead to complete bearing failure.

Even if the bearing itself is not affected by these discharges, its lubrication could be. The grease composition can degrade rapidly under the effect of current discharges, with the high localized temperatures generated causing the lubricant’s additives and base oils to react, with burning or charring of the oil (Figure 2).

The cause
At the root of all these problems, according to Geoff Brown, drives application consultant with motor and drive manufacturer ABB, New Berlin, Wisc., is what is known as “common mode voltage.” Under normal conditions, a typical three-phase sinusoidal power supply is balanced and symmetrical — the vector sum of the three phases is always equal to zero, with the neutral at zero volts. ASDs, however, work by converting that sinusoidal line AC voltage to DC, then back to a pulse-width-modulated (PWM) AC voltage of variable frequency, by which the motor speed can be controlled. The switching frequency of these pulses can range from 1 kHz to 20 kHz and, while the voltages may be balanced in peak amplitude, this variation makes it impossible to achieve perfect balance between the phases instantaneously. When this happens, the neutral is no longer zero but at what can be defined as a common mode voltage.

“Common mode voltage induces current in the motor shaft,” explains Brown, “and it is this ‘common mode current’ that causes the problem.” Again, it is not a new problem. Brown harks back to the early days of offshore oil and gas production rigs in the North Sea, where large 3.3-kV systems generally were not grounded and the problem of stray currents became something of a cause c<accent acute>él<accent grave>èbre. “Basically,” he says, “it was found that if you didn’t have a balanced supply then you would end with circulating currents.”

That ASDs can cause circulating currents has clearly been known for some time, but the problem seems to have become exacerbated by the very improvements that have made ASDs increasingly popular — namely, the fast-switching insulated gate bipolar transistor (IGBT) technology commonly employed in the drives. As Brown’s colleague, Matti Laitinen, drives design manager of ABB Industry Oy, Helsinki, Finland, explained: “Fast rising voltage pulses produced by modern power supplies contain high frequencies that initiate high frequency currents which will flow through stray capacitances in the motor system. Such currents are part of the total common mode current, and follow a path called the common mode loop. Several such loops can be formed in any drive system, depending on the drive system architecture and the installation techniques used, but they all start at the source of the common mode voltage, the inverter itself.” Here the fluctuating potential of the DC bus produces large current flows at very high pulse frequencies, and these currents will seek the path of least resistance to return to the bus — via the windings, the shaft and, if they are not stopped or diverted, via the bearings themselves.